Method of manufacturing near-net shape alloy product

ABSTRACT

A method of producing a heat treatable metal product having the ordered steps of providing an ingot of a heat treatable metal alloy; rolling the ingot to a flat product; removing material from the product to achieve a near-net shape of a desired final shape; and solution heat treating the product. The product is stretched to the desired final shape and aged to achieve the desired mechanical properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for manufacturing metal alloyplate product in a near-net shape prior to heat treatment. Moreparticularly, the invention relates to the production of a metal alloyproduct in which a metal alloy plate or sheet is produced in dimensionsthat are similar to the desired final dimensions of the product, thenheat treated, stretched and aged.

2. Description of Related Art

Aluminum alloys are extensively used in aircraft and automobiles as wellas other products that benefit from the unique combination of propertiessuch as low weight (compared to ferrous alloys), high strength, fracturetoughness, and corrosion resistance. Aircraft and automotive structuralcomponents are typically produced from a sheet or plate to whichadditional structural members are attached. For example, the upper andlower wing panels of an aircraft are typically produced from aluminumalloy plate (referred to as the skin) which is reinforced by extrudedcomponents (referred to as stringers). The stringers are fixed to theskin by fasteners such as rivets.

Aluminum aircraft components produced from flat rolled products aretraditionally manufactured from direct chill cast ingots that may beseveral feet thick. The ingot is then hot-rolled to a preliminary plateor sheet thickness. Subsequent cold rolling may be performed prior toheat treating, stretching and artificial or natural aging. Prior to theartificial aging step, the mechanical properties of the alloy such asyield strength and fracture toughness are improved by solution heattreating the alloy at elevated temperatures which alters themicrostructure of the solute components in the alloy. The metal is thenquenched to lock in the microstructure of the alloy achieved duringsolution heat treatment. After heat treatment, the material isstretched, artificially or naturally aged and machined or chemicallymilled to its final shape.

Some final products (e.g. aircraft wing ribs) have relatively thickcross-sections of up to ten inches thick that are machined from plate.For these thick gauge applications, the ingot is hot-rolled, solutionheat treated and quenched. Rapid quenching immediately followingsolution heat treatment is desirable for rapid locking of the elementsneeded for strengthening in the microstructure. A slow quench rate risksloss of mechanical properties. However, the quench rate is dependent onthe plate thickness. As the thickness of the plate increases, the quenchrate for the plate decreases which results in lower achievablemechanical properties. Moreover, some aluminum alloys have mechanicalproperties that are readily lost if rapid quenching is not performed. Itwould be desirable to produce such alloys in thick cross-sectionsutilizing high quench rates to take advantage of improved mechanicalproperties. However, the product thickness has been limited by thequench sensitivity of those alloys and slower quench rates. Increasingthe quench rate or reducing the cross section thickness, will result inimproved mechanical properties

Accordingly, a need remains for a method of producing heat treatablemetal product at gauges at which the quench rate is acceptable.

SUMMARY OF THE INVENTION

This need is met by the method of present invention for producing a heattreatable metal product. According to one aspect of the invention, aningot of a heat treatable metal alloy such as aluminum alloy in the Ftemper (the temper of the alloy as fabricated) is rolled to a flatproduct. Material from the flat product is removed to achieve a shapesimilar to the desired final (net) shape, referred to herein as thenear-net shape. The near-net shaped product is solution heat treated andstretched. The stretching step flattens, stress relieves and brings theshape to the desired final dimensions. The stretched product is thenartificially aged. Suitable alloys for use in the present invention arealuminum alloys including alloys of the Aluminum Association (AA) series2XXX, 6XXX and 7XXX. The final product may be an aircraft component suchas a wing panel or an automotive component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a component produced according to thepresent invention;

FIG. 2 is a plot of electrical conductivity for the lower surfaces ofplates produced according to the present invention and control plates;

FIG. 3 is a plot of electrical conductivity for the upper surfaces ofplates produced according to the present invention and control plates;

FIG. 4 is a plot of tensile strength for plates produced according tothe present invention and control plates;

FIG. 5 is a plot of fracture toughness for plates produced according tothe present invention and control plates, both in the T7651 temper; and

FIG. 6 is a plot of fracture toughness for plates produced according tothe present invention and control plates, both in the T7451 temper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention includes a method of producing heat treatablemetal products having a superior combination of properties and economiesof production. The invention may be used with various alloy systemsincluding aluminum, magnesium, copper and other alloys which rely onheat treatment for precipitation of solute for strengthening purposes,referred to herein as a “heat treatable metal alloy”. The presentinvention is especially useful for AA 2XXX, 6XXX and 7XXX seriesaluminum alloys.

According to the present invention, a heat treatable metal alloy productis produced as an ingot by direct chill casting or the like. The termingot is meant to include other bulk metal products. The metal alloyingot is rolled into a flat product. Material from the flat product isremoved resulting in a shape of the product that is near-net to adesired final shape. By the phrase “near-net shape” it is meant that theproduct is dimensionally similar to the dimensions of the desired finalshape. Material removal may be accomplished by various techniquesincluding via machining. The near-net shaped product is then solutionheat treated and quenched. Solution heat treatment may be performed inany type of solution heat treat furnace using either a spray or animmersion quench system. The orientation of the product in a solutionheat treat furnace may be selected to account for potential quenchdistortion and equipment design.

By removing material so that the product achieves a near-net shape, theproduct is thinner and the quench rate for the product is higher thanthe quench rate for an ingot or a traditional flat product. Accordingly,superior properties are achievable in a product produced according tothe present invention due to the opportunity for high quench rate.

The solution heat treated product is then stretched to achieve thedesired final dimensions for the product. The phrase “final dimensions”refers to the dimensions of the product after practicing the presentinvention, which may not necessarily be the ultimate dimensions of acompleted component. Further altering of the product dimensions mayoccur depending on the end use for the product. Stretching of a productreduces the residual stresses in the product, provides flatness to theproduct and may improve metallurgical properties of the product incertain alloys. By removing material so that the product achieves anear-net shape prior to solution heat treatment, the stretching processis facilitated due to the smaller volume of the product being stretched.Typically, stretching decreases the dimensions of the stretched productby about 1.25% in each of the product thickness and the product width,i.e. has a target stretch of about 2.5%. The step of removing materialso that the product achieves a near-net shape should take into accountthe decrease in the thickness and width of the product duringstretching. For example, the dimensions of the product in its near-netshape should be 1.25% larger in each of the product thickness and theproduct width so that the stretching step results in the desired thefinal dimensions.

Finally, the stretched product which is in its desired final shape isaged such as via artificial aging. Traditionally, artificial aging isperformed on a plate or sheet product. By artificially aging a productproduced according to the present invention which has a significantlythinner cross-section than conventional aged products, the artificialaging process may be performed more rapidly in both the heat up of theproduct and the cool down of the product providing greater control andconsistency.

Although the invention has been described generally above, theparticular examples give additional illustration of the product andprocess steps typical of the present invention.

EXAMPLES

Two ingots of Aluminum Association alloy 7085 each weighingapproximately 19,000 pounds were randomly selected from existingcommercial stock. The ingots referred to as Ingot A and Ingot B werechosen from different casting campaigns. Alloy 7085 has the compositionlimits shown in Table 1. TABLE 1 ELEMENT MIN MAX SILICON — 0.06 IRON —0.08 COPPER 1.3 2.0 MANGANESE — 0.04 MAGNESIUM 1.2 1.8 CHROMIUM — 0.04ZINC 7.0 8.0 TITANIUM — 0.06 ZIRCONIUM 0.08 0.15 OTHER IMPURITIES, EACH— 0.05 OTHER IMPURITIES, — 0.15 TOTAL ALUMINUM REMAINDER

Ingots A and B were scalped and homogenized using standard fabricationpractices and procedures. Both ingots were hot rolled into plates 3.5inches thick by 74 inches wide by 600 inches long using standard hotrolling temperatures and pass schedules. Each of the two 3.5 inch×74inch plate pieces were cut in half along its length to yield two sets oftwo 300 inch long plates. One plate from Ingot A and one plate fromIngot B were processed according to the present invention to produceplates having integrally formed stringers on a web (referred to as ISPplates). The two ISP plates were machined from the F temper down thefull length to produce the cross section configuration shown in FIG. 1and are referred to as ISP A plate and ISP B plate. The other platesfrom each of Ingots A and B were processed in their originalconfigurations as control plates and are referred to as Control A plateand Control B plate.

All four plates were solution heat-treated in a horizontal furnace andspray water quenched in accordance with AMS (American Metals Society)2772 guidelines. The ISP plates A and B were solution heat treated withthe stringers oriented up. All plates were stretched to a target of 2%.

To check for quench efficiency, the plates were scanned using eddycurrent to evaluate the surface electrical conductivity uniformity. FIG.2 shows the % IACS (International Annealed Copper Standard) electricalconductivity across the width (longitudinal transverse direction) ofbottom sides (lower surface during quenching) of the ISP A and B plates(smooth side, without the machined stringers) and of the Control platesA and B. FIG. 3 shows the % IACS electrical conductivity for the topsides of the plates (upper surfaces during quenching). The data for thetop side of the ISP plates was taken on stringers at the edge of theplates (Edge Stringer) and on stringers midway across the plates (W/2Stringer). The data for the top side of the control plates was taken athalfway across the width of the plates (W/2). For the bottoms of theplates and the top sides of the plates, the % IACS electricalconductivity for the ISP plates was uniformly lower than for the controlplates. Lower % IACS electrical conductivity is indicative of higherquench rate. As such, product with thinner cross sections thanconventional products can be produced according to the present inventionat higher quench rates.

Prior to artificial aging the four plate sections, each plate was againcut in half to yield a piece of plate approximately 150 inches long. The150 inch plates were aged to each of T7651 type temper and T7451 typetemper. Two control plates and two ISP plates were aged to each tempercondition. Upon completion of the artificial aging, the plates weresectioned for mechanical property testing. To ensure an accuratecomparison between the control plate properties and the ISP plateproperties, special care was taken to keep all of the test planereferences relative to the original rolled plate thickness. This wasimportant to eliminate variability in test results associated withchemical composition or grain structure gradients through thickness.Tensile strength testing was performed on the webs of the ISP andcontrol plates from each of Ingots A and B at a plane halfway throughthe thickness of the plate web (t/2) at width locations of the plateedge (Edge) and midway across the plate (W/2). Fracture toughnesstesting was performed at planes of halfway through the thickness of theplate web (Web/2), one quarter through the thickness of the stringer(t/4) and halfway through the thickness of the stringer (t/2) at severallocations across the plate including the plate edge (Web Edge), thestringer edge (Stringer Edge), midway across the plate in the web (WebW/2), and midway across the plate in the stringer (Stringer W/2). Testlocations for the control plates were made in the locations of the solid(non-machined) plates as if the control plates had been machined toinclude stringers and web so that the comparison between ISP and controlplates was referenced from the same position in the original ingot.

FIG. 4 shows the longitudinal transverse tensile strength data in KSIunits (ultimate yield strength or UTS and tensile yield strength or TYS)for the ISP and control plates produced from Ingots A and B, aged toT7651. The ISP plate strengths are generally equal to or slightly abovethe control group.

FIG. 5 shows the fracture toughness for the ISP and control platesproduced from Ingots A and B aged to T7651, and FIG. 6 shows similardata for plates aged to the T7451 temper. The ISP plates generallyexhibited 10% to 20% higher fracture toughness than the control platesin all locations for both tempers.

It should be appreciated that the present invention allows for machining(or other material removal) of bulk metal into a near-net shape prior tosolution heat treatment, stretching and aging with superior propertiesin the final product compared to conventionally manufactured productthat is machined after aging.

The present invention provides economical benefits in the production ofaluminum alloy products. Productivity may be increased due to higherquench rates and shorter aging practices of the near-net shape productwith concomitant energy savings.

In addition, certain aluminum alloy products with complicated shapeshave not previously been produced from flat-rolled wrought products.Extrusion, forging and casting processes are typically employed tomanufacture such products and those processes do not always impart themechanical properties achievable with wrought products. However, byusing the method of the present invention the shapes of extruded, forgedor cast products may be manufactured in a wrought product by removingmaterial from a flat-rolled (wrought) product to achieve a near-netshape prior to solution heat treatment.

Further potential benefits of the present invention include reducedweight of components and manufacturing cost savings for the end usecustomer. The total weight for structural components may be reduced bymanufacturing product according to the present invention. Fasteners andother related structural members required for joining components to eachother are minimized or avoided when the product is machined to itsnear-net shape as an integrally manufactured product. For example thestiffening members (e.g. stringers) of an aircraft wing panel may beintegrally produced with the skin using the method of the presentinvention. The opportunity for cost savings to the end use customer mayinclude reduced process time to achieve the final part configuration(e.g. less machining or chemical milling time) and reduced processscrap.

It will be readily appreciated by those skilled in the art thatmodifications may be made to the invention without departing from theconcepts disclosed in the foregoing description. Such modifications areto be considered as included within the following claims unless theclaims, by their language, expressly state otherwise. Accordingly, theparticular embodiments described in detail herein are illustrative onlyand are not limiting to the scope of the invention which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

1. A method of producing a heat treatable metal product comprising theordered steps of: (a) providing an ingot of a heat treatable metalalloy; (b) rolling the ingot to a flat product; (c) removing materialfrom the product to achieve a shape near-net to a desired final shape;(d) solution heat treating the product; (e) stretching the product toachieve the desired final shape; and (f) aging the product.
 2. Themethod of claim 1, wherein the metal alloy is an aluminum alloy.
 3. Themethod of claim 1, wherein step (c) comprises machining the product toachieve the near-net shape.
 4. The method of claim 2, wherein thealuminum alloy is selected from the group consisting of AA series 2XXX,6XXX and 7XXX.
 5. The method of claim 2, wherein the flat product is inthe F temper.
 6. The method of claim 2, wherein the final product is acomponent of an aircraft.
 7. The method of claim 6, wherein thecomponent is a wing panel.
 8. The method of claim 7, wherein step (c)comprises machining the near-net shape of a skin and stiffening membersin the wing panel.
 9. A heat treated metal component produced accordingto the method of claim
 1. 10. The heat treated metal component of claim9, wherein said product is a component of an aircraft.
 11. The heattreated metal component of claim 10, wherein the component is a wingpanel.